1. Field of the Invention
The present invention relates to a receiver capable of receiving an analog broadcast and a digital broadcast and an integrated circuit (IC) for the same.
2. Description of Related Art
In the current amplitude modulation (AM) broadcast and the current frequency modulation (FM) broadcast, the processing of materials to be broadcast is being digitized. However, the broadcasts themselves are performed in an analog system, hereafter the analog system broadcast will be called as an xe2x80x9canalog broadcastxe2x80x9d.
However, if the broadcasts themselves are digitized, a broadcast in high quality can be supplied, and additional data service can be enabled, and further the efficiency of frequency utilization can be improved. Accordingly, a digital broadcast, in which broadcasts themselves are digitized, is now under consideration.
As one of the digital broadcast system, DAB, namely, a digital audio broadcast in conformity with Eureka 147 standard is employed in Europe. On the other hand, Integrated Services Digital Broadcast for Terrestrial (ISDB-T) is proposed in Japan.
Incidentally, the ISDB-T is a standard employing the following specifications in a case of a narrow-band ISDB-T and mode 1 so that digital audio data of a plurality of channels and other digital data can simultaneously be broadcast.
transmission bandwidth: 436 kHz
carrier interval: 4 kHz
carrier number: 109
carrier-modulating system: 16QAM, 64QAM, QPSK, DQPSK
multiplex system: MPEG2
information rate: 283.1 kbps-1651.4 kbps
In addition, the digital broadcast is scheduled to be mixed in the same frequency band as that of the analog broadcast, especially the FM broadcast. For example, in Europe and Japan, a vacant channel in the current VHF television broadcast is scheduled to be used for the digital broadcast.
Now, as a receiver for the analog broadcast, there is proposed a receiver that performs the frequency conversion of a received signal to intermediate-frequency signals of in-phase and quadrature, the phases of which are orthogonal to each other, and that eliminates image signals by executing phase-shift processing and operation processing of the intermediate-frequency signals (see e.g. IEEE Transactions on Consumer Electronics, Vol. 38, No. 3, Aug. 1992, pp. 465-475).
Because the intermediate-frequency can be lowered in such a structure, an intermediate-frequency filter can be made to be an IC, and therefore an entire receiver can be made in one chip IC.
Accordingly, it can be conceivable that a receiver for the digital broadcast is similarly structured. However, it is required for a receiver of the digital broadcast to have a disturbance-rejection ability more superior than that of a receiver of the analog broadcast. In addition, if a receiver that eliminates image signals in conformity with the aforementioned method is mass-produced, the limit of the ability of the elimination of the image signals is 35 dB-40 dB under the current semiconductor technique. This image rejection characteristic is not sufficient for the receiver of the digital broadcast.
Accordingly, there is also considered a receiver employing a receiving system in which the intermediate frequency is made to be zero at the time of frequency conversion, i.e. the direct conversion system, so that the image signals themselves are not generated. However, in a broadcast wave signal of the analog broadcast, a carrier signal has much energy, and if the carrier signal is eliminated, large distortion is generated. Consequently, the direct conversion system has problems in direct current amplification and a direct current offset.
Furthermore, the digital broadcast occupies wider bandwidth than that of the analog broadcast. In addition, because the digital broadcast uses a vacant channel of the current VHF television broadcast in Europe and Japan, a strong television broadcast wave exists adjacently to the digital broadcast wave. And further, as mentioned above, the requirement for the disturbance-rejection ability for the receiver of the digital broadcast is stricter than that for the receiver of the analog broadcast.
The present invention aims to enable a single receiving circuit to receive both an analog broadcast and a digital broadcast, and to realize the receiving circuit in an IC. In addition, the present invention aims to resolve the aforesaid problems on the occasion of realizing the receiving circuit.
According to a first aspect of the present invention, there is provided a receiver for receiving an analog broadcast and a digital broadcast, the receiver comprising: a mixer circuit for performing a frequency conversion of a received signal to an intermediate-frequency signal by means of a local oscillation signal; a filter circuit for taking the intermediate-frequency signal out of an output signal of the mixer circuit; and a demodulation circuit for taking a signal indicating broadcast contents in the analog broadcast or the digital broadcast out of the intermediate-frequency signal taken out by means of the filter circuit, wherein the receiver is constructed so that, at a time of receiving the analog broadcast, a frequency of the local oscillation signal is apart from a carrier frequency of the received signal by an intermediate frequency of the intermediate-frequency signal and that, at a time of receiving the digital broadcast, the demodulation circuit takes the signal indicating broadcast contents in the analog broadcast out of the intermediate-frequency signal, and the receiver is constructed so that the frequency of the local oscillation signal is equal to a center frequency of the received signal and that the demodulation circuit takes the signal indicating broadcast contents in the digital broadcast out of the intermediate-frequency signal.
According to a second aspect of the present invention, there is provided a receiver for receiving an analog broadcast and a digital broadcast, the receiver comprising: a first mixer circuit and a second mixer circuit for performing a frequency conversion of a received signal to a first intermediate-frequency signal and a second intermediate-frequency signal, respectively, phases of which are orthogonal to each other by means of a local oscillation signal; a first filter circuit and a second filter circuit for taking the first intermediate-frequency signal and the second intermediate-frequency signal out of output signals of the first mixer circuit and the second mixer circuit, respectively; and a demodulation circuit for taking a signal indicating broadcast contents in the analog broadcast or the digital broadcast out of the first intermediate-frequency signal and the second intermediate-frequency signal taken out by means of the first filter circuit and the second filter circuit, wherein the receiver is constructed so that, at a time of receiving the analog broadcast, a frequency of the local oscillation signal is apart from a carrier frequency of the received signal by an intermediate frequency of the first intermediate-frequency signal and the second intermediate-frequency signal and that the demodulation circuit takes the signal indicating broadcast contents in the analog broadcast out of the first intermediate-frequency signal and the second intermediate-frequency signal, and the receiver is constructed so that, at a time of receiving the digital broadcast, the frequency of the local oscillation signal is equal to a center frequency of the received signal and that the demodulation circuit takes the signal indicating broadcast contents in the digital broadcast out of the first intermediate-frequency signal and the second intermediate-frequency signal.
According to a third aspect of the present invention, there is provided an IC for receiving an analog broadcast and a digital broadcast, the IC comprising: a mixer circuit for performing a frequency conversion of a received signal to an intermediate-frequency signal by means of a local oscillation signal; and a filter circuit for taking the intermediate-frequency signal out of an output signal of the mixer circuit, wherein the IC is constructed so that, at a time of receiving the analog broadcast, a frequency of the local oscillation signal is apart from a carrier frequency of the received signal by an intermediate frequency of the intermediate-frequency signal and that, at a time of receiving the digital broadcast, the frequency of the local oscillation signal is equal to a center frequency of the received signal and further that a signal indicating broadcast contents in the analog broadcast or the digital broadcast out of the intermediate-frequency signal taken out by the filter circuit.
According to a fourth aspect of the present invention, there is provided an IC for receiving an analog broadcast and a digital broadcast, the IC comprising: a first mixer circuit and a second mixer circuit for performing a frequency conversion of a received signal to a first intermediate-frequency signal and a second intermediate-frequency signal, respectively, phases of which are orthogonal to each other by means of a local oscillation signal; and a first filter circuit and a second filter circuit for taking the first intermediate-frequency signal and the second intermediate-frequency signal out of output signals of the first mixer circuit and the second mixer circuit, respectively, wherein the IC is constructed so that, at a time of receiving the analog broadcast, a frequency of the local oscillation signal is apart from a carrier frequency of the received signal by an intermediate frequency of the first intermediate-frequency signal and the second intermediate-frequency signal and that, at a time of receiving the digital broadcast, the frequency of the local oscillation signal is equal to a center frequency of the received signal and further that a signal indicating broadcast contents in the analog broadcast or the digital broadcast out of the first intermediate-frequency signal and the second intermediate-frequency signal taken out by the first filter circuit and the second filter circuit.
Accordingly, the present invention provides a receiver and an IC in which an analog broadcast is received through a superheterodyne system and a digital broadcast is received through a direct conversion system.
According to the aforesaid aspects of the present invention, an image rejection characteristic and an adjacent disturbance wave characteristic necessary for receiving an analog broadcast or a digital broadcast can be obtained. In addition, the intermediate frequency can be lowered. Consequently, the receiving circuit including an intermediate-frequency filter can be made to be an IC. Moreover, almost all of the circuits can be shared in a case of installing them in an IC, and thereby it can be realized to miniaturize the shape thereof and to lower the price thereof. Furthermore, when an analog broadcast is received, there is no need to consider DC amplification and a DC offset. Besides, when a digital broadcast is received, there occurs no problem owing to the DC offset.